To elaborate on meikl's answer, from the Borland Community (just use Edit instead of Memo):

The Windows TEdit and TMemo controls have no overwrite capability. It is possible to emulate this behavior however, by setting the SelLength property of the edit or memo control to one during the processing of the KeyPress event. This causes the character at the current position of the caret to be overwritten.

The following example demonstrates emulation of an overwrite capability of a TMemo component. The state of the overwrite mode can be toggled by pressing the insert key.

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